Particulates or particulate matter from a diesel engine is mainly constituted by carbonic soot and a soluble organic fraction (SOF) of high-boiling hydrocarbon and contains a trace of sulfate (misty sulfuric acid fraction). In order to suppress such kind of particulates from being discharged to atmosphere, it has been carried out as shown in FIG. 1 that a particulate filter 6 is incorporated in an exhaust pipe 4 through which exhaust gas 3 from a diesel engine 1 flows.
In the example shown, exhaust gas 3 discharged from an automobile's diesel engine 1 (internal combustion engine) via an exhaust manifold 2 flows through an exhaust pipe 4 with a muffler 5 which receives therein catalytic regenerative particulate filter 6 integrally carrying oxidation catalyst, the particulate filter 6 being encased by a filter casing 7 which forms an outer cylinder of the muffler 5.
More specifically, as shown in enlarged scale in FIG. 2, the muffler 5 has inlet and outlet pipes 8 and 9 between which is secured a required size of reception chamber 12 defined by and between dispersion plates 10 and 11 and with a number of communicating holes 10a and 11a, respectively. The particulate filter 6 is in the reception chamber 12.
As schematically shown in section in FIG. 3, the particulate filter 6 is constituted by a porous honeycomb structure made of ceramics and having lattice-like compartmentalized passages 6a; alternate ones of the passages 6a have plugged inlets and the remaining passages 6a with unplugged open inlets are plugged at their outlets. Thus, only the exhaust gas 3 passing through thin porous walls 6b compartmentalizing the respective passages 6a is discharged downstream.
The particulates, which are captured by and accumulated on inner surfaces of the walls 6b, require to be appropriately burned off so as to regenerate the particulate filter 6 before exhaust resistance increases considerably due to clogging. However, the exhaust gas from the diesel engine in a normal operating status rarely has a chance to reach a temperature level at which the particulates spontaneously ignite. Thus, catalytic regenerative particulate filter 6 integrally carrying oxidation catalyst has been developed for practical use, said oxidation catalyst being for example platinum-alumina catalyst added with an appropriate amount of rare-earth element such as cerium.
Adoption of such catalytic regenerative particulate filter 6 accelerates oxidation reaction of the captured particulates to lower their ignition temperature, so that the particulates can be burned off at an exhaust temperature lower than ever before.
However, even if such catalytic regenerative particulate filter 6 is adopted, an accumulated particulate amount may exceed a treated particulate amount in an operation region with lower exhaust temperature. When such operation status with lower exhaust temperature continues, there may be a fear that regeneration of the particulate filter 6 does not proceed well, disadvantageously resulting in excessive capturing of particulates by the particulate filter 6. In order to overcome this, it has been considered that, when the accumulated particulate amount becomes increased, fuel is added to the exhaust gas 3 upstream of the particulate filter 6 so as to forcibly regenerate the filter 6.
More specifically, addition of the fuel upstream of the particulate filter 6 causes the added fuel to effect oxidation reaction on the oxidation catalyst of the filter 6; reaction heat generated therefrom increases the catalyst floor temperature to burn off the particulates, thereby regenerating the particulate filter 6.
This kind of forcible regeneration of the particulate filter 6 is disclosed, for example, in the following References 1 and 2.    [Reference 1] JP2003-155915A    [Reference 2] JP2003-222040A
In an exhaust emission control device as shown in FIG. 4 which includes a flow-through type oxidation catalyst 13 (see FIG. 5) upstream of the particulate filter 6 in the reception chamber 12 so as to especially accelerate the oxidation reaction of the captured particulates, the added fuel is oxidized at the oxidation catalyst 13 upstream of the particulate filter 6 to generate reaction heat; the exhaust gas 3 elevated in temperature by the reaction heat is introduced into the particulate filter 6. Thus, forcible regeneration of the particulate filter 6 can be accomplished even at further lower exhaust temperature.
JP 2003/214136 describes an exhaust emission control device in which a particulate filter is mounted in the middle of an exhaust pipe, an oxidation catalyst layer is carried on a surface which is in contact with an exhaust gas in sheet metal parts forming a flowing space of the exhaust gas at a more upstream side than the particulate filter.
JP 2003/106139 describes an exhaust emission control device in which a catalyst-regenerative type particulate filter is provided in a filter case installed in the middle of an exhaust pipe, a distribution board having a large number of communication holes is opposedly arranged close to the inlet-side end face of a particulate filter in the filter case and a part of a communication holes of a distribution board is formed in the bell mouth shape whose diameter is gradually decreased toward the inlet side at face of the particulate filter to be a nozzle part for throttling the flow of an exhaust gas.